In liquid crystal display (LCD) devices, such as those used in laptop computers and flat panel televisions, an image is formed by manipulating liquid crystal material disposed between a substrate and a glass cover at discrete points on the display to selectively pass light through the liquid crystal material. At each discrete point, an individually-controllable electro-optical element that defines a pixel of the image is created by forming a common electrode on the substrate and patterning a pixel electrode on the glass cover. The liquid crystal material reacts in response to the electric field established between the common electrode and pixel electrode to control the electro-optical response of the pixel.
For example, the pixel electrodes in LCD devices are typically driven by a matrix of thin film transistors (TFTs). Each TFT individually addresses a respective pixel electrode to load data representing a pixel of an image into the pixel electrode. The loaded data produces a corresponding voltage on the pixel electrode. Depending on the voltages applied between the pixel electrode and the common electrode, the liquid crystal material reacts at that electro-optical element to either block or transmit the incoming light. In some applications, the pixel electrodes can be driven with voltages that create a partial reaction of the liquid crystal material so that the electro-optical element is in a non-binary state (i.e., not fully ON or OFF) to produce a “gray scale” transmission of the incoming light.
A traditional illumination device that is used in color LCD devices is a backlight unit that provides a uniform field of light to each of the electro-optical elements in the display. The backlight unit may be illuminated by red, blue and green light emitting diodes (LEDs) that are mixed to produce white light. However, the light intensity of LEDs degrades differently over time. Therefore, some LCD devices include an optical feedback system that measures the degradation of each LED and compensates for the LED degradation by adjusting the intensity of each LED, for example, by pulse width modulation of the LED drive current. Typically, an optical sensor fitted with a color filter is positioned adjacent the backlight unit to measure the intensity of light produced by each LED.
However, the color sensors available on the market today are typically complicated and expensive. In addition, measuring the light in the backlight unit does not take into account any changes in the spectral content resulting from the light passing through the liquid crystal material. Therefore, what is needed is a display device including a low cost, simple optical feedback system that compensates for degradation of the light due to the LCD.